Project Summary Asthma and obesity are public health crises that have concurrently arisen over the past decades, affecting millions of children in the US and disproportionately affecting low-income minority children in urban areas. The same children at highest risk for asthma and obesity also have greater exposure to pollution. Emerging evidence suggests that these three factors ?asthma, obesity, and pollution? may be causally linked. Recent observational studies have found that obesity confers susceptibility to pulmonary effects of particulate matter (PM), including our own showing overweight Baltimore children with asthma have greater respiratory response to indoor PM versus lean children. There is a need to confirm these observational findings with causal-level evidence, including experimental evidence of the effect of being overweight on PM susceptibility and identification of plausible mediators of this effect. To provide experimental evidence, project 1 will conduct a randomized, clinical trial of an air purifier intervention to address our overarching hypothesize that overweight children are more susceptible to PM compared to lean children. Portable air purifiers have been used successfully in clinical trials of inner-city children, reduce indoor PM by ?50%, and reduce asthma symptoms. Moreover, air purifiers are a highly feasible, inexpensive intervention that can be implemented immediately to modify indoor environments. To define plausible mediators of effect, we will investigate four leading hypotheses that increases in 1) tidal volume and thereby increases in particulate dose to the lungs 2) systemic inflammation and oxidative stress that may result in exaggerated responses to pro-inflammatory PM exposure 3) glucocorticoid resistance which may dampen the protective effect of inhaled steroids 4) risk of obstructive sleep apnea (OSA), a comorbidity recently linked to PM exposure that is also associated with worsening of asthma symptoms, enhance susceptibility to PM among overweight versus lean children. The study findings will be critical to future revisions of air quality standards and have high potential to impact policy by defining a new sensitive population to pollution health effects. Further, our findings will immediately impact asthma clinical practice guidelines by providing evidence to support a highly feasible intervention for the highest risk subpopulation of children with asthma, a population easily identifiable by height and weight measurements. Moreover, as our target population is low-income, minority children, our findings will have implications for those most affected by both obesity and asthma.